Cars pass under a neon tsunami warning following an earthquake off the coast of northeastern Japan early on Saturday. Photo: AP

Workers at the crippled Fukushima nuclear plant were evacuated when a small tsunami hit Japan after a powerful undersea earthquake yesterday, highlighting the continued threat to the area devastated by the 2011 quake-tsunami.

The magnitude-7.1 earthquake struck at 2.10am off the coast of northeastern Japan and its epicentre was located off Fukushima prefecture at a depth of 10 kilometres, the Japan Meteorological Agency said.

The quake prompted the agency to issue a tsunami warning and thousands of residents were urged to evacuate in the prefectures of Iwate and Miyagi. Ships left a port south of the Fukushima nuclear power station as a precaution after the tremor. Buildings shook as far away as Tokyo, 230 kilometres south of the nuclear plant.

"It was fairly big and rattled quite a bit, but nothing fell to the floor or broke. We've had quakes of this magnitude before," said Satoshi Mizuno, a disaster management official with the Fukushima prefectural government. "Luckily, the quake's centre was very far off the coast."

Mizuno said the operator of the troubled Fukushima plant, Tokyo Electric Power Co (Tepco), reported that no damage or abnormalities had been found.

Two workers patrolling wells used to measure underground water at Fukushima sought higher ground after the tremor struck, a Tepco official said, adding that few workers were working on the waterfront at the time.

Another nuclear plant, at Onagawa, was the site of the largest wave recorded yesterday - 55cm - but there were no problems reported there. All of Japan's 50 nuclear reactors remain offline as the government decides whether they meet more stringent requirements enacted after the 2011 quake.

The meteorological agency said the quake was an aftershock of the March 2011 tremor. "There is the possibility that aftershocks with a magnitude of around seven will occur once in a while," Keiji Doi, director of the agency's quake prediction team. The tsunami advisory was lifted less than two hours after the quake.

The area affected largely overlapped with that hit by the March 2011 disaster when more than 18,000 people died after a towering tsunami crashed ashore following a 9.0-magnitude undersea quake.

In the town of Ofunato, a 20cm tsunami was logged just after 3am, while Ishinomaki, which was devastated in 2011, recorded a 30cm wave.

"We evacuated as a matter of precaution because the ground floor of our house was flooded in the tsunami two years ago," Chimaki Hojyo, a 69-year-old housewife in Ofunato, told the Yomiuri newspaper.

"This kind of tsunami will keep us worried."

Eastern Japan was struck by a 6.5-magnitude earthquake last month causing tremors that were felt 600 kilometres away in Tokyo.

Although Tepco says Fukushima's reactors are now under control, critics say the plant remains in a precarious state and at the mercy of extreme weather or further quakes. They point out that there is still no plan for the thousands of tonnes of water being stored on site.

Tens of thousands of people remain in temporary accommodation, with some scientists warning that it could be decades before they are able to return home - if at all.

Nuclear Regulatory Authority chairman Shinichi Tanaka has scheduled a meeting tomorrow with Tepco's president to seek solutions to what he says appear to be fundamental problems.

Agence France-Presse, Associated Press, The New York Times

This article appeared in the South China Morning Post print edition as Tsunami threat forces Fukushima evacuation

Why no mention of the plan to remove the spent fuel rods manually? What kind of devastation would occur if this 7 magnitude aftershock had happened two weeks from now after this process had begun or when another one happens during this year long process? Articles like this are irresponsible in failing to connect the TEPCO dots that are growing larger every day.